The present invention relates, generally, to the art of oxygen sensors. In particular, the present invention relates to heating an oxygen sensor via multiple heating elements each having a different resistance value.
Oxygen sensors are typically employed in a vehicle's exhaust system to sense oxygen so that the fuel air ratio of exhaust gasses emanating from the engine can be calculated by an engine controller. The sensors are placed in the exhaust system of the vehicle, one upstream and one downstream of the catalytic converter, so that the operating efficiency of the catalytic converter may also be monitored via the engine controller. The oxygen sensors have a specific operating temperature range, and may not detect proper amounts of oxygen prior to reaching this range given inherent sensor limitations. Therefore, it is desirable to have the sensors quickly heated, after start-up, to within the operating temperature range thereby allowing for peak operation and oxygen detection.
It has been demonstrated that the oxygen sensors will begin to warm given the heat generated by the engine exhaust. Since the exhaust is relatively cool upon start of the engine, however, the sensors are slow to reach operating temperature range if only the heat of the exhaust is relied upon to heat the sensor. The industry has tried to remedy this by equipping oxygen sensors with a low resistance heating element that is in communication with the inner core element of the oxygen sensor. Since heater performance is inversely proportional to the resistance, the lower the resistance, the quicker the oxygen sensor heater will reach its desired temperature operating range. If the heater is too low in resistance, however, the heater will ramp to high temperature that is out of its operating range. As a result, the inner core element of the sensor may crack or otherwise have a degradation in performance by not giving accurate readings to the engine controller.
Still other oxygen sensor systems supply maximum electrical power to the single heater for a set time after start-up and regulate the power thereafter. This requires the use of costly circuitry for regulating voltage supplied to the heater. Moreover, complex wave shaping circuitry is also required for shaping current waveforms supplied to the sensor heater.
It is therefore desirable in the art of oxygen sensors to have a sensor that heats up quickly to with its temperature operating range and then levels off at a constant temperature for all operating conditions thereafter and that does not require the use of costly, complex voltage and current regulation circuitry.